1. Field of the Invention
The present invention relates to an organic electroluminescent device.
2. Description of the Related Art
Organic electroluminescent devices are provided with a thin film containing a light-emitting organic compound between an anode and a cathode. By injecting holes and electrons from the respective electrodes, excitons of the light-emitting organic compound are generated. When these excitons return to a ground state, the organic electroluminescent devices irradiate light.
Recent developments in organic electroluminescent devices have been very significant. Characteristic examples of such developments include high brightness at low applied voltages, more diversity in emission wavelengths, rapid response, and the ability to produce thinner and lighter light-emitting devices. As a result of these developments, a broad range of possible applications are being suggested for organic electroluminescent devices.
However, especially when considering applications in full color displays, present devices do not have sufficient luminous efficiency or stability for practical use. Especially for blue-light-emitting devices, there is a need for further improvements in performance.
Regarding improving the stability of organic electroluminescent devices, for example, Japanese Patent Application Laid-Open No. 2005-108726 describes a device which is characterized by containing an electron mobile host as a blue-light-emitting layer, a hole mobile material as an assist dopant, and also a light-emitting dopant. Further, Japanese Patent Application Laid-Open No. 2006-156888 describes an example in which, as the light-emitting layer, an electron trapping dopant and a hole trapping dopant, which have a smaller energy gap than the host material, are mixed in an anthracene host material.
In conventional blue-light-emitting devices, the recombination probability of the holes and the electrons in the light-emitting layer, charge trapping, and the expansion of the emission region in the light-emitting layer are insufficient, whereby sufficient luminous efficiency and drive durability could not be obtained. Further, the energy transfer efficiency of the light-emitting dopant for conventional blue-light-emitting devices has been insufficient, so that there have been problems with the emitted color and luminous efficiency.